This application requests funds to collect retrospective questionnaire data on an estimated 50,000 births in the reproductive histories of 8,000 twins in the Norwegian Twin Registry and their adult offspring. The multiple unique genetic relationships that exist within these families will be exploited using a new model for the analysis of quantitative inheritance in man, that will permit for the first time a clear resolution of the effects of genetic environmental and maternal factors on the outcome of pregnancy. The causes of subfertility, abortion, miscarriage and fetal death will be investigated as well as birth weight, complications of pregnancy, and the occurrance of congenital malformations. The collected data will permit a critical test of the multifactorial model for the inheritance of common birth defects, and could lead to the discovery of new causes of specific birth defects.